Many important optical materials exhibit birefringence. Birefringence means that different polarizations of light travel at different speeds through the material. These different polarizations are most often considered as two components of the polarized light, one being orthogonal to the other.
Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces or fields. Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam traversing the sample. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit a linearly birefringent sample with a phase difference, called the retardance. If the incident light beam is circularly polarized, two orthogonal components of the polarized light will exit a circularly birefringent sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm). An “average” birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample.
The need for precise measurement of birefringence properties has become increasingly important in a number of technical applications. For instance, it is important to specify and control the residual linear birefringence (hence, the attendant induced retardance) in optical elements used in high precision instruments employed in semiconductor and other industries. The optics industry thus has a need for a highly sensitive instrument for measuring linear birefringence in optical components. This need has been largely unmet, especially with respect to measurements of low levels of retardance.
Linearly polarized light may also be characterized as the superposition of two components of circularly polarized light (right-hand and left-hand senses) having identical amplitude and frequency or wavelength. The relative phases of the two circular polarization components determines the polarization plane. The plane of polarization will be rotated in instances where the refractive indices of a sample are slightly different for the two senses of circular polarization. This rotation of the polarization plane is referred to as optical rotation. Optical rotation is also referred to as circular birefringence because it relates to the phase shifting of the circular polarization components that is attributable to the different refractive indices.
If linearly polarized light passes through chiral media, such as, for example a solution of chiral molecules, the polarization of the incident light will be rotated. This circular birefringence (or optical rotation) is often referred to as natural optical rotation to distinguish it from Faraday rotation in a magnetic field. The extent of optical rotation, therefore, is indicative of the molecular structure (the chirality) of such media. Thus, the precise detection and analysis of the optical rotation, or circular birefringence, imparted by sample of chiral medial is useful for analytical chemistry, pharmaceutical, and biological industries.
The complete description of each linear and circular birefringence requires two parameters. Both linear and circular birefringence of a sample along a given optical path require specifying the magnitude of the birefringence, or the amount of integrated phase retardation along the given optical path length. For linear birefringence, it is also required to specify the fast axis of the sample. The two orthogonal polarization components described above are parallel to two orthogonal axes, which are determined by the sample and are called the “fast axis” and the “slow axis.” The fast axis is the axis of the material that aligns with the faster moving component of the polarized light through the sample. For circular birefringence, the sense of optical rotation, in terms of clockwise or anticlockwise, should be specified. Therefore, a complete description of the linear and circular birefringence of a sample along a given optical path requires specifying both the magnitude of the birefringence, the relative angular orientation of the fast (or slow) axis, and the rotational direction (for circular birefringence).
The present invention is directed to a practical system and method for precisely measuring low-level linear and circular birefringence properties of optical materials. The retardance magnitude and orientation of the fast axis are precisely calculated, as well as the rotational direction. The system permits multiple measurements to be taken across the area of a sample to detect and graphically display variations in the retardance across the sample area.
In a preferred embodiment, the system incorporates a photoelastic modulator for modulating polarized light that is then directed through a sample. The beam propagating from the sample is separated into two parts. These separate beam parts are then analyzed at different polarization directions, detected, and processed as distinct channels. The detection mechanisms associated with each channel detect the light intensity corresponding to each of the two parts of the beam. This information is employed in an algorithm for calculating a precise, unambiguous measure of the retardance induced by the sample and the orientation of the fast axis.
As one aspect of this invention, the system includes a beam-splitting member and detector arrangement that permits splitting the beam into two parts with minimal contribution to the retardance induced in the beam. Moreover, the presence of any residual birefringence in the optical system (such as may reside as static birefringence in the photoelastic modulator or in any of the optical components of the system) is accounted for in a number of ways. For example, certain of the system components are arranged or mounted to minimize the chance that strain-induced birefringence may be imparted into the element. A reliable calibration technique is also provided.
The system permits the low-level linear and circular birefringence measurements to be taken at any of a plurality of locations across the area of the sample. The measurements are compiled in a data file and graphically displayed for quick analysis.
In one embodiment of the invention, the optical components of the system are arranged to measure the birefringence properties of a sample that is reflectively coated on one side, thereby permitting measurement of birefringence properties even though the sample is not completely light transmissive.
Other advantages and features of the present invention will become clear upon study of the following portion of this specification and drawings.